Lekarz powiedział, że witamina D i odpoczynek też są ważne.

Powiedział is the past tense, masculine singular form of the verb powiedzieć (to say / to tell).

• lekarz (doctor) is a masculine noun
• so the verb in the past tense matches it: lekarz powiedział

Compare:

• Lekarz powiedział = The doctor said (male doctor, or doctor treated grammatically as masculine)
• Lekarka powiedziała = The female doctor said

Also, powiedzieć is a perfective verb, so it suggests a completed act of speaking: he said.

Że means that and introduces a subordinate clause.

So the structure is:

• Lekarz powiedział = The doctor said
• że witamina D i odpoczynek też są ważne = that vitamin D and rest are also important

In Polish, że is very common after verbs like:

• powiedzieć = to say
• myśleć = to think
• wiedzieć = to know
• uważać = to believe / consider

English often omits that, but Polish usually keeps że.

Because the subject is compound: witamina D i odpoczynek = vitamin D and rest.

That means there are two things, so the verb must be plural:

• jest = is (singular)
• są = are (plural)

So:

• Witamina D jest ważna. = Vitamin D is important.
• Odpoczynek jest ważny. = Rest is important.
• Witamina D i odpoczynek są ważne. = Vitamin D and rest are important.

Ważne is the plural non-masculine-personal form of ważny (important).

In Polish, plural adjectives have different forms depending on whether the group is:

• masculine personal (at least one male person) → e.g. ważni
• non-masculine-personal (things, ideas, animals, women-only groups in many contexts) → e.g. ważne

Here the subject is:

• witamina D (a thing)
• odpoczynek (a thing/concept)

So Polish uses the non-masculine-personal plural:

• są ważne

Compare:

• Lekarze są ważni. = Doctors are important.
  (male-person or mixed-person group)
• Witaminy są ważne. = Vitamins are important.

Because it is the subject of the clause, so it is in the nominative case.

In this part:

• witamina D i odpoczynek też są ważne

both witamina D and odpoczynek are subjects of są ważne.

That is why they appear in their dictionary/basic forms:

• witamina
• odpoczynek

If they had another role in the sentence, their forms might change. For example:

• Potrzebuję witaminy D. = I need vitamin D.
  (witaminy D = genitive)

Też means also / too.

In this sentence, it tells us that vitamin D and rest are also important, probably in addition to something mentioned earlier.

Its position is fairly flexible, but placement changes what sounds most natural or what gets emphasis.

Here:

• witamina D i odpoczynek też są ważne

This is natural and means something like:

• Vitamin D and rest are also important

You may also see:

• Też witamina D i odpoczynek są ważne
• Witamina D i odpoczynek są też ważne

These can work in context, but the original version sounds very natural.

Yes, Polish word order is more flexible than English word order, because grammatical endings carry a lot of information.

The neutral version here is:

• Lekarz powiedział, że witamina D i odpoczynek też są ważne.

But other orders are possible for emphasis, for example:

• Lekarz powiedział, że też witamina D i odpoczynek są ważne.
• Lekarz powiedział, że ważne są też witamina D i odpoczynek.

These are not all equally neutral. The original sentence is a very natural, standard way to say it.

So the word order is not random, but it can shift depending on:

• emphasis
• style
• rhythm
• what information is new vs. already known

Yes. Odpoczynek is a very normal noun meaning rest.

It is related to the verb:

• odpoczywać = to rest

Examples:

• Potrzebujesz odpoczynku. = You need rest.
• Odpoczynek jest ważny. = Rest is important.

In health-related advice, odpoczynek is exactly the kind of word you would expect.

Good question. Witamina by itself is feminine singular, so you would say:

• Witamina D jest ważna.

And odpoczynek is masculine singular, so:

• Odpoczynek jest ważny.

But when you join them with i (and), the subject becomes plural:

• witamina D i odpoczynek

Now the adjective and verb must agree with the combined subject, not with just one noun:

• są ważne

So the singular genders of the individual nouns matter less than the fact that together they form a non-masculine-personal plural subject.

I is the normal Polish word for and.

In this sentence:

• witamina D i odpoczynek = vitamin D and rest

It simply joins two nouns.

A pronunciation note: i is pronounced like ee in see.

Also, after some words, Polish may use a instead of i, but a usually suggests more contrast, while i is the basic additive and.

Że is pronounced roughly like zheh.

More precisely:

• ż sounds like the s in measure or the zh sound in vision
• e is like e in met (though Polish vowels are pure and shorter)

So:

• że ≈ zheh

It is a very common word, so it is worth getting comfortable with quickly.

Yes: D is simply the name of the vitamin, just as in English.

So:

• witamina D = vitamin D

The letter name in Polish is pronounced differently from English, but in writing it stays D.

Also, the whole phrase behaves grammatically like a noun phrase headed by witamina, so:

• Witamina D jest ważna.
• Potrzebuję witaminy D.

The witamina part changes for case; the letter D stays the same in writing.

Yes, in many contexts you could say doktor, but lekarz is the more basic and standard word for doctor in the sense of a medical doctor.

• lekarz = doctor / physician
• doktor can also mean someone with a doctoral degree, depending on context

So in a straightforward health sentence, lekarz is a very natural choice.

The pattern is:

• [subject] + [past-tense verb] + że + [subordinate clause]

Specifically:

• Lekarz = subject
• powiedział = past-tense verb
• że = that
• witamina D i odpoczynek też są ważne = subordinate clause

Inside the subordinate clause:

• witamina D i odpoczynek = subject
• też = also
• są = are
• ważne = important

So it is a very useful structure for reported speech:

• X powiedział, że... = X said that...